# NTRVL: Norwegian 4x4 Trainer — VO2 Max Interval Timer + HR > VO2 Max Interval Timer + HR (iPhone/iPad app by Andrii Gryshutkin.) - Source: https://appshunter.io/ios/app/ntrvl-norwegian-4x4-trainer/id6768314931 (this page in markdown: same URL + `.md`) - Developer: [Andrii Gryshutkin](https://appshunter.io/developer/1884593015) - Category: Health & Fitness, Lifestyle - Price: Free with in-app purchases - Age rating: 4+ - Requires: iOS 17.0 · 5 MB - Languages: American English - Released: 2026-05-16 - Data updated: 2026-09-06 - User reviews in markdown: https://appshunter.io/ios/app/ntrvl-norwegian-4x4-trainer/id6768314931/reviews.md ## What is NTRVL? Four hard four-minute intervals. Three minutes easy in between. That is the Norwegian 4x4, and NTRVL runs the whole thing for you. 10-minute warm-up. Four intervals of 4 minutes in zone 4, with 3 minutes of active recovery. 5-minute cool-down. NTRVL counts every phase, tells you when to push and when to ease off, and saves the session to Apple Health. HEART RATE, HOWEVER YOU HAVE IT - AirPods Pro 3 or Powerbeats Pro 2: train with just your phone and your earbuds - Apple Watch: start on your wrist and leave the phone at home - No sensor at all: NTRVL runs the protocol by feel, on effort alone Pull your AirPods out mid-interval and the watch takes over the heart rate without stopping the workout. START ON EITHER DEVICE Whichever device you start on runs the session. The other becomes a live screen you can control from either side. Phone-first or watch-first, both work. TWO NUMBERS AFTER EVERY SESSION - Zone accuracy: the share of each interval you actually spent in zone 4, interval by interval. - Heart rate recovery: how far your pulse falls in the first 60 seconds after a hard interval. It moves within a few weeks, so it is the earliest honest sign the training is landing. ZONES CALCULATED FOR YOU NTRVL works out your zones with the Karvonen heart-rate-reserve method over a Tanaka maximum, using your age, resting heart rate and biological sex from Apple Health. Tested your max HR in a lab? Enter it by hand and NTRVL uses that instead. AN 8-WEEK PROGRAMME, NOT A STOPWATCH Three sessions a week, 24 in all. NTRVL tells you where you are in the block and when the next one is due. BUILT FOR TRAINING, NOT FOR STARING AT A SCREEN - Voice cues at every phase change, with your music ducked - Haptics on the watch for every phase and every zone change - Live Activity on the Lock Screen and in the Dynamic Island - Skip the warm-up when you are already warm, or let it run - Run, bike, row, ski-erg, stairs. The session is written to Apple Health as that activity, with heart-rate samples and energy burned ABOUT VO2 MAX VO2 max is a measure of aerobic fitness: how much oxygen your body can use at full effort. It is trainable, and interval work is one of the most studied ways to improve it. The 4x4 protocol was developed at NTNU in Trondheim, Norway, and is among the most researched interval sessions there is. PRIVACY Your heart-rate data never leaves your iPhone. No account, no server, no analytics on your health. Everything is processed on device and stored in Apple Health, where you own it. TRY THE PROTOCOL FREE One full short session, a single interval of about 12 minutes, free. No card, no account. NTRVL Pro unlocks the full four-interval protocol, the 8-week programme, recovery trend, training calendar, unlimited history, PDF export for your coach, and custom interval lengths and counts. Pro is 4.99 USD per month, or 29.99 USD per year with the first week free. REQUIREMENTS - iPhone running iOS 17 or later - Optional for heart rate: AirPods Pro 3, Powerbeats Pro 2, or Apple Watch Series 4 or later - The Apple Watch app can run a full session on its own, with or without the phone nearby NTRVL is a training tool, not a medical device, and does not provide medical advice. High-intensity training is not right for everyone. Talk to a doctor before starting a new training programme. ntrvl.app Terms of Use (EULA): https://www.apple.com/legal/internet-services/itunes/dev/stdeula/ Privacy Policy: https://www.ntrvl.app/privacy ## Pricing and in-app purchases Base price: Free. **Subscriptions** | Subscription | Price | Period | | --- | --- | --- | | NTRVL Pro Yearly | $29.99 | per year | | NTRVL Pro Monthly | $4.99 | per month | ## Version history (last 5 releases) ### 1.3 — 2026-09-05 THE DONE BUTTON ON THE SUMMARY SCREEN Only the word Done responded to your tap. The orange button around it did nothing, so finishing a workout meant hitting a small target, sometimes over and over. The whole button works now. YOUR RECOVERY NUMBER, ON THE FREE SESSION TOO The short first session was meant to give you both numbers — how much of the interval landed in zone 4, and how fast your heart came back down. It could only ever give you the first one. Now it gives you both. WORKOUTS REACH APPLE HEALTH, AND AS THE RIGHT KIND Training by feel, with no AirPods and no watch, is a real workout. It now gets written to Health like any other, so it counts toward your Activity rings. Sessions with a heart-rate sensor carry that heart-rate data across as well. If you picked Bike or Row and wore your watch, Health recorded it as a run. It now records what you actually chose. REMINDERS THAT ACTUALLY ARRIVE The reminder for your next session was being scheduled for a time that had already passed, so it never appeared. It works now, and the next few sessions are queued — missing one no longer ends the chain. CLEARER FROM THE FIRST SCREEN The intro said forty minutes and then offered you twelve, with nothing connecting the two. It now tells you the first session is short. And the start button says Start Workout for everyone, instead of naming the sensor you happen not to own. APPLE WATCH - A session you finished on the watch is recorded as finished, not abandoned. - The heart-rate sensor is no longer left running after the phone goes quiet. - The phone was waking the watch app every second during a workout — over two thousand times across a single session. Together with the item above, this was the battery drain some of you reported. - Workouts the watch had not managed to hand over could pile up, and then all arrive at once, each one taking over the summary screen. They now go quietly into your history, and the watch clears them properly once the phone confirms it has them. - Starting from the phone no longer opens two workout sessions at once. - The Start button on the watch could get stuck refusing to start, silently. Fixed, and if it ever does refuse it now says why. SMALLER THINGS - Turning the phone sideways during a sensor-free session no longer shows an empty heart-rate dial. - Ending a session early no longer offers you a locked export. There is nothing to export from a workout you stopped, and we should not be asking. ### 1.2 — 2026-09-04 TRY IT IN TWELVE MINUTES There is now a free short session: one interval, about twelve minutes, with the warm-up and cool-down included. It is the real thing, once instead of four times — long enough to feel what a 4x4 interval is and to get your first heart rate recovery number. No subscription, and no heart rate hardware required. The full forty-minute protocol is one tap away when you want it. We also stopped showing the upgrade screen before you have finished a single session. There is nothing worth selling until you have seen what the app does. APPLE WATCH - The short session now runs correctly when you start it from your phone. It used to fall back to the full forty minutes. - Your phone and your watch now show the same number of intervals. - Battery: the watch no longer keeps a workout session or its timers running after a workout has ended, and no longer re-queues finished workouts every time you switch apps on your phone. - If your phone stops responding mid-session, the watch tells you and gives you a way out instead of freezing. APPLE HEALTH The permission screen now says the thing Apple's own screen does not: it opens with every switch turned off, and tapping Allow without turning them on grants nothing. If access is off, the main screen says so and links straight to Settings. ALSO FIXED - Done on the summary screen sometimes needed several taps. - The monthly average no longer counts sessions where heart rate was never measured. One session by feel used to drag it down by a third. - An exported PDF no longer prints a large 0% for a session that had no heart rate data. ### 1.1 — 2026-08-31 Train with what you have. START FROM YOUR APPLE WATCH Your watch can now run the whole session on its own — heart rate from your wrist, phases, haptics and the summary. Leave the phone at home if you like. Start from either device and the other one follows along, and says which is which. NO AIRPODS? NOT A PROBLEM If your AirPods come out mid-workout, your watch takes over as the heart rate sensor without stopping the session. And with no sensor at all, Train by Feel runs the full 4x4 protocol on the clock and tells you how hard each phase should feel. AN EIGHT-WEEK PROGRAM 24 sessions, three a week — the dose the research actually used. Miss a day and nothing breaks; the schedule just shifts. TWO NUMBERS AFTER EVERY SESSION How much of the work landed in the right zone, and how fast your heart came back down in the first minute. The second one usually improves in two to three weeks — long before VO2 Max moves. RECOVERY TREND (PRO) Your recovery week over week, so you can see whether the line is climbing. ALSO IN THIS RELEASE - Live Activity on the lock screen: phase and time left without unlocking. - The workout screen can no longer get stuck when the watch goes out of range. - Battery fixes on Apple Watch: the workout session and its timers now always shut down. - Clearer subscription terms before you buy. ### 1.01 — 2026-08-27 This update adds privacy-first usage analytics: anonymous and aggregated, never linked to your identity, never used for tracking or advertising. It shows us where NTRVL gets in the way — heart rate not connecting, for example — so we can fix what actually breaks. Your workouts and heart rate data stay on your device and in Apple Health. ### 1.0 — 2026-05-16 No release notes. ## More apps by Andrii Gryshutkin - [Heart Sit](https://appshunter.io/ios/app/heart-sit/id6760467612) - [KettlePulse](https://appshunter.io/ios/app/kettlepulse/id6761713734) - [Tactu: Pain Journal](https://appshunter.io/ios/app/tactu-pain-journal/id6762459710) - [Latibule](https://appshunter.io/ios/app/latibule/id6764277888) All apps by Andrii Gryshutkin: https://appshunter.io/developer/1884593015 --- *Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-09-06.*